In the prior art, a variety of substances including metal particles, ferrites, carbon or graphite particles, oxides of the metals zinc, germanium, barium, tin, iron and the like have been incorporated into coatings for producing heat in a microwave oven, i.e. to act as a heating susceptor for the purpose of absorbing a portion of the microwave energy and converting it to heat. Various other chemical susceptors such as salts are employed in an aqueous solution for this purpose as described in U.S. Pat. No. 4,283,427. A quantity of free water must be provided to dissolve the salt so that it is in an ionic form that will interact with the microwave energy to produce heat. This requires that the wet product be placed in a pouch that is sealed at its edges. This wet product has many disadvantages including its bulk, fluidity and the complexity of the manufacturing operation. U.S. Pat. Nos. 4,264,668 and 4,518,651 describe coatings containing carbon black. However, it has been found that carbon-containing heat producing coatings, when heated in a microwave oven, can be subject to a runaway heating condition that often produces arcing, sparking, burning or charring of the backing sheet to which they are applied. U.S. Pat. Nos. 4,806,718; 4,808,780; 4,810,845 and 4,818,831 describe ceramic devices for microwave heating, primarily green ceramics, which employ a quantity of bound water to produce heating. The ceramic gel itself produces heat.
In developing the present invention it was found that when carbon was used alone with a film former, such as a standard ink base, that burning and uncontrolled temperature rise occurs. Many of the packages burst into flames when heated in a microwave oven. It was also found that when carbon was mixed with an aqueous acrylic dispersion, the resulting susceptor would burn the package. A rapid, uncontrolled temperature rise occurs. Discoloration appears at about 400.degree. F. Then ignition follows almost immediately. The package starts to brown at about 400.degree. F. and then quickly begins to burn which is, of course, unacceptable. Once the package begins to carbonize, this facilitates further heating and accelerates the burning reaction which causes burning to occur at an even faster rate. This can be referred to as runaway heating.
An important objective of the invention is to provide a microwave susceptor layer that can be applied at little or no pressure as a fluid and which, upon exposure to microwave heating, will produce a uniform heat without unacceptable arcing, popping, sparking or burning. It is another objective to obtain uniformity of heating in different portions of the package and also from one sample to another. The susceptor composition should have characteristics that allow it to be applied as a fluid by a variety of methods including roll printing, silk screen printing, spraying, dipping, brushing and the like. The composition should preferably be useful with gravure printing, one application method found to allow especially good coating weight control. The fluid susceptor, sometimes referred to herein for convenience as "ink," should be capable of being applied directly onto a backing such as paper, paperboard or the like without the requirement for multiple superimposed coatings, plastic sheets or high pressure which increase production costs and capital requirements.
When applied by printing, the fluid susceptor composition should have all the qualities of a good printing ink including the proper rheological properties: viscosity, dilatency and thixotropy to avoid problems such as misting, splattering or dripping from freshly printed surfaces moving at high speed and must also transfer easily from the supply roll to the printing roll. The susceptor fluids or inks of the present invention should also produce coatings of uniform thicknesses and be able to form both a continuous and interrupted coating, e.g. a coating with a multiplicity of openings or uncoated spots within a coated area.
It is a further object of the invention to control or stabilize the heat produced by a microwave interactive material by providing a cooling effect at a selected temperature or at a plurality of temperatures within a selected temperature range to compensate for the heat produced by the microwave interactive material.
A more specific object is to control heating of a susceptor so that it can be used on paper without the paper charring or catching on fire.
When printing is the application method, another object is to enable printing of the susceptor to be accomplished using standard printing equipment at normal speeds, up to 1200 feet per minute. A further object is to provide a susceptor for heating foods which is food safe.
Yet another object is to match or exceed the performance of commercially available microwave susceptors that employ vapor deposited semiconductive aluminum coatings.
When overheating occurs at the periphery or along the edge of a susceptor, it is an object to reduce or eliminate overheating, charring or burning of this kind along the edge of a printed susceptor.
These and other more detailed and specific objects of the invention will be apparent in view of the accompanying drawings and specification which set forth by way of example but a few of the various forms of the invention that will be apparent to those skilled in the art once the principles described herein are understood.